High-frequency agitation sources, such as piezoelectric crystals, are well known in the art and are used for a number of purposes. Piezoelectric motors, transformers and linear drives are common. An important use for a piezoelectric crystal is in nebulisation. There are many cases where a fine mist of a substance is required without the application of heat. One example of this is a medical nebuliser, wherein a pharmaceutical compound is nebulised by a piezoelectric crystal in order to be inhaled by a patient. Another use for nebulisers is in the field of water dispersal such as garden water features.
A problem with piezoelectric crystals is that, in operation, they can generate a large amount of thermal energy. A piezoelectric crystal under constant operation may get very hot if appropriate measures to sink the thermal energy (such as heat sinks) are not provided. Piezoelectric crystals are prone to damage at high temperatures so it is desirable that the temperature of the piezoelectric crystal does not become excessive.
When forming part of a nebuliser, a piezoelectric crystal acts on a head of liquid in order to disperse the liquid into a fine mist. During operation of the piezoelectric crystal, the head of liquid absorbs the vibrational energy and sinks some of the thermal energy of the piezoelectric crystal. This has the effect of cooling the piezoelectric crystal. However, if the piezoelectric crystal continues to operate when all of the liquid has been nebulised, the temperature of the crystal will rapidly increase. This may lead to thermal damage. Further, it is desirable that unnecessary use of the piezoelectric crystal (which can be wasteful of energy) is avoided.
Prior art methods to deal with this problem are illustrated in U.S. Pat. No. 4,001,650 and U.S. Pat. No. 5,803,362. U.S. Pat. No. 4,001,650 discloses the use of a detector to detect surface motion of liquid in the nebuliser. When no surface motion is detected, the liquid is deemed to have been completely evaporated and the nebulisation process is stopped. However, the arrangement of U.S. Pat. No. 4,001,650 requires complicated detectors.
U.S. Pat. No. 5,803,362 discloses a temperature control device which is capable of varying the power fed to an oscillator circuit depending upon the temperature of a piezoelectric crystal. This process can prevent the temperature of a piezoelectric crystal from exceeding a maximum temperature. However, varying the power supplied to (and thus the amplitude of oscillation of) a piezoelectric crystal can be an inefficient method of controlling a piezoelectric crystal.